The evidence has been mounting for years that early humans and Neanderthals interbred, but now…
Read more Read more

Two new papers, one in Nature and the other in Science, suggest that Neanderthal genes helped European and Asian peoples' ancestors adapt to the colder environments outside of Africa. Twenty percent of the Neanderthal genome lives on in humans today, and non-African people generally have 1 to 4% Neanderthal DNA.

Genes that researchers identified as uniquely Neanderthal are involved in the body's production of hair and skin. Perhaps Neanderthal hair kept our ancestors warmer, and thus they passed the valuable genes on. And possibly Neanderthal skin was better suited to the northern weather, or better adapted to local microbes. But we simply don't know for sure.

It seems quite compelling that as modern humans left Africa, met Neanderthals, and exchanged genes, we picked up adaptive variants in some genes that conferred an advantage in local climatic conditions . . . Unfortunately, skin and hair do so many things that it's hard to speculate on what specifically that adaptive trait was.

What is clear is that these genes were beneficial, because they were preserved in many people's genomes for so long.

Many of the Neanderthal genes may also have created problems, including male infertility. The New York Times' Carl Zimmer spoke with both teams of researchers. He writes:

Both teams of scientists also found long stretches of the living human genomes where Neanderthal DNA was glaringly absent. This pattern could be produced if modern humans with certain Neanderthal genes couldn't have as many children on average as people without them. For example, living humans have very few genes from Neanderthals involved in making sperm. That suggests that male human-Neanderthal hybrids might have had lower fertility or were even sterile.

Overall, said Dr. [David] Reich, "most of the Neanderthal genetic material was more bad than good."

This also speaks to a question that often comes up in discussions about Homo sapiens having children with Neanderthals. Given that the two groups of humans had diverged over 600,000 years before they met again, might their offspring not be sterile, like mules? Obviously, many of the hybrid offspring were not sterile, since their genes remain alive in modern humans today. But perhaps many male hybrids were infertile, partly because — as Reich and his team put it in their Nature paper — "Neanderthal alleles caused decreased fertility in males when moved to a modern human genetic background."

In light of genetic evidence, anthropologists today do not always define Homo sapiens and Neanderthals as separate species. But the groups were different enough that their offspring were not always able to reproduce. Still, it seems that the Neanderthals left a visible genetic legacy behind among non-Africans. Both Europeans and Asians owe their hair and skin to Neanderthal ancestors.